Recently, accompanying improvements in the performance of image forming apparatuses such as digital copiers, integrated digital devices having not only a copying function, but also a function as a printer have been developed and popularized. In such an image forming apparatus, even more added value with respect to the respective functions have been required.
Namely, in the image forming apparatus, an image formed on a recording medium is fixed by a fixing device such as a fixing roller, and is discharged. With respect to the temperature control of the fixing roller here, there are various prior arts.
In Jpn. Pat. Appln. KOKAI Publication No. 2001-185338, “Induction Heating Device and Image Processing Device Equipped with Same” is disclosed, and there is disclosed a technique in which a plurality of induction coils are connected in parallel, and the current supplies to only some coils and to the plurality of coils are respectively switched by switching operations, and a desired temperature distribution can be obtained.
Further, in Jpn. Pat. Appln. KOKAI Publication No. 2002-40872, there is disclosed a technique in which the end portions and the central portion of a heating roller in an induction heating type fixing device are detected, and by varying the switching frequency in accordance with the temperature difference thereof, the temperature distribution of the heating roller is made to be uniform.
Furthermore, in Jpn. Pat. Appln. KOKAI Publication No. 2001-126856, there is disclosed a technique in which, in an induction heating type fixing device, a temperature difference between a temperature detection at the central portion and temperature detection at the end portions, in the longitudinal direction, of a fixing roller is provided for controlling, whereby it is possible shorten a rise time.
However, in these prior arts, two-split coils are alternately energized (heated) at intervals of every given time with reference to a data table of a given heating ratio. Further, in a high frequency IH control, a high-temperature side as well is heated for the given time, thus irregularities in temperature result. In addition, there is the problem that because there is a limit to a minimum switching time due to the problem of a control circuit, overshooting and undershooting arise.